Michael Keating, a professor of politics at the University of Aberdeen, said the example set by Catalonia showed that referendums held by regional authorities without the backing of central government were little more than "super opinion polls".
The academic's argument goes against the case made yesterday by veteran SNP MSP Alex Neil, who claimed that a so-called ‘wildcat’ independence referendum organised by the Scottish Government without legal authority under the Scotland Act would not be invalidated by a Unionist boycott.
Boris Johnson last week confirmed he would not considering granting permission for a second referendum, and his Scottish Secretary Alister Jack has claimed even a pro-independence referendum at the 2021 Scottish Parliament election wouldn’t change the position.
Mr Neil - a former Scottish Government cabinet minister - argued that MSPs already had the power to hold a consultative referendum that would change the facts on the ground if it is won by the pro-independence side.
But Prof Keating, who leads the Centre on Constitutional Change, said such a consultative vote would only be a "political gesture".
"The legality is not at all clear - what is clear is that the Union is a reserved matter," he said during an interview with BBC Good Morning Scotland.
"Scotland can't change that. Whether you're allowed to have a consultative referendum, which is a really super opinion poll on a reserved matter, is something that has never been tested.
"But fundamentally it's not a legal matter, because even if such a referendum were to go ahead it would only be a political gesture, it's not going to have any affect.
"Besides which, the No side, the Unionist side could simply boycott it and deprive it of any meaning - so we would be in a situation as they were in Catalonia where they had a referendum, there was 90 per cent in favour of independence, but the turnout was only just over 40 per cent.
"So even as a political gesture it could backfire.”